If you're ready, head into the cave. You'll want to explore as much as possible, because you truly need the training. You will be fighting Pokémon Level 55 and higher at the Pokémon League! That is why using Repels is not recommended— all the battles will force you to train enough. Be aware that once you cross the first bridge (you need to get to the ladder in the northwest corner to get through Victory Road), Wally will battle you.
If you're truly lost...
If you are truly, hopelessly lost, here is the simplest path through Victory Road: On the ground floor, go north to the second platform of stairs and across the bridge (Wally will show up to battle you here) and down the ladder in the northwest corner. Go over the bridge on B1F, down the stairs and then south a bit. Around the corner, there is a ladder. Go down it to B2F. Surf across the water, climb the waterfall and Surf around to go down the other waterfall. Surf to the bit of land with the ladder on it. Climb the ladder and go south and around a corner to the east. Climb the ladder that you find, and you'll be back on the first floor. Go up the stairs out of the little crater you were in, across the bridge, and down the other stairs. Go north as far as you can, and to the east is the exit.
Ever Grande City (northern half)
The straightforward path at the end of Victory Road leads to an impressive building at the top of the city, where your final challenge awaits! The next section of this walkthrough will cover your battles against the Elite Four and Champion.